Known biological gas purification systems comprise a biowasher, wherein the micro-organisms are dispersed in the liquid, and a so-called biotrickling filter (abbreviated below to BTF), wherein the micro-organisms ore situated at least partially on the packing material.
In the case of a biowasher the micro-organisms degrade the contamination absorbed from the gas flow by the liquid. The micro-organisms herein grow and the quantity of biomass thereby increases. This results eventually in draining of excess biomass.
In the case of a BTF the micro-organisms grow on the packing material in the form of a biofilm. The contaminants from the liquid are absorbed into the liquid which drips along the biofilm and diffuse into the biofilm. The micro-organisms degrade the contaminants and the degradation products such as water, carbon dioxide and occasionally mineral salts or acids, are relinquished to the liquid. This liquid can moreover contain nutrients for the micro-organisms in addition to acid or alkali to buffer the liquid to a neutral pH.
Due to excessive growth the biofilm can increase in size such that this results in an increased pressure drop over the BTF and ultimately even to blocking of the BTF. All kinds of mechanical and chemical procedures are applied to remove at least a part of the biofilm. The micro-organisms are for instance poisoned to enable easier mechanical removal thereof. As a result of the fact that the BTF must be taken temporarily out of service for maintenance, it is necessary to install two BTF installations to enable continuous biological cleaning of a gas flow. An additional problem is that the released biofilm forms waste which has to be processed and discharged separately.